Process and apparatus for the decomposition of alkali metal amalgams



Feb. 3, 1959 R. CRABBE ET AL PROCESS AND APPARATUS FOR THE DECOMPOSITION OF ALKALI METAL AMALGAMS Filed Dec. 20. 1955 ifiw w mflw wwww a m E E w! 9 @9 9 9 1 f 1 5 1O 7|WU l IF l 1' 1 H H 8 8 8 8 8 8 8 J 5 A.|.li!|.|.l.l.|i|.l.l. 6 F E 5 W49 mmw mmm 7 I I iw J! 11 11 1 1 1 8 8 8 8 8 8 8 5 M PROCESS AND APPARATUS FOR THE DEcoMro- SITION F ALKALI METAL AMALGAMS 8 Claims. (Cl. 23-184) The invention relates to the manufacture of caustic lyes of high concentration in alkali metal hydroxides by decomposition of the corresponding metal amalgams in the presence of water. It is an object of the invention to provide a particular process for the decomposition of these I amalgams.

The speed of decomposition of alkali metal amalgams varies as a direct function of their concentration and as an inverse function of the corresponding hydroxide concentration of the aqueous solution with which they are in contact. For this reason it has often been proposed to circulate the amalgam in decomposition apparatus, called denuder cells or simply denuders, in opposite direction to the circulation of the caustic lyes being formed in the assembly of consecutive compartments forming the denuder. On the other hand, it is known that the manufacture of concentrated caustic lyes is preferably performed from amalgams having a relatively high alkali metal content. This concentration cannot, however, exceed a definite limit without seriously disturbing the working of the amalgam producing cell. The alkali metal concentration required may easily be attained in a modern cell or, as has been proposed (in German patent application W 9092), by deriving from the outlet of the cell an aliquot part of the amalgam and introducing it through the inlet of the cell together with the mercury stripped of its alkali metal after passage through the decomposer.

The decomposition apparatus or denuders of the horizontal type commonly used effect a counter-current or equi-current circulation of the amalgam and of the lye produced. In the vertical denuders of usual construction, the idea of a well defined circulation is not so clear and precise. Actually, these vertical denuders are formed by scrubbers of a more or less developed type containing a contact mass capable of accelerating the, decomposition of the amalgam, and completely filled with the electrolyte. Needless to say, because of the liberation of hydrogen due to the reaction of the alkali metal amalgam with the water of the lye, vvan intimate mixing is produced which has the tendency of rendering uniform the concentration of the said lye. The mixing action of the gas is further augmented by the fact that the concentrated lye, i. e. having the highest specific gravity, is drawn oif towards the top of the apparatus.

.In any case, whatever" principle of circulation may be adopted for the amalgam and the lye, two concentration gradients are encountered at the same time in the amalgam and in the lye. At the point of the decomposition apparatus where the alkali metal concentration of the amalgam is highest there may be found the lye richest in alkali metal hydroxide (counter-current circulation) or the most dilute lye (equi-current circulation) or viceversa. It is possible that the amalgam is not completely decomposed unless the apparatus is sufliciently largewhich to some extent involves the risk of bad Working of the electrolytic cell or, moreover, a rather unreasonable utilisation of the volume of the decomposition apparatus.

In actual denuders all of the amalgam feed is introduced 7 nited States atent at one point only. It .is the object of the present invention to provide modifications of this working method so as to overcome some inconveniences inherent in the aforesaid processes, especially with regard to the manufacture of concentrated caustic lyes whilst using apparatus of the most rational dimensions and eliminating, if need be, the risks involved by the presence of too highly concentrated amalgams in the electrolytic cells.

The process according to the invention consists in dividing the mercury feed containing the amalgam to be decomposed into several fractions and in simultaneously introducing these fractions into the denuder at points arranged at a distance apart along the passage of the caustic lye being formed, this introduction being effected in such a manner that at no instant is the amalgam to be decomposed mixed with amalgam already partially or wholly decomposed. The amalgam is thus distributed among several compartments which constitute together the decomposition denuder; the caustic lye being formed circulates consecutively through each of these compartments and is periodicallyput in contact with fresh amalgam during its travel until the required alkali metal hydroxide concentration is obtained and the said lye leaves the denuder. In contrast to this the mercury containing the amalgam does not pass from one compartment to the next. At the outlet of each of these compartments it is taken up by. a collector which according to whether or not the amalgam is completely decomposed conducts the said mercury either to the manufacture of rich amalgam or towards a'denuder. which may be called a second step denuder where the incompletely decomposed amalgam gives up the alkali metal.

.This phase of the process must not be confused with one of .the modifications of the process described in Belgian Patent No. 482,217 according to which the amalgam consecutively passes through two distinct decomposition apparatus. According to the process of the present invention, the caustic lye of relatively low concentration formed in the second step denuder constitutes the electrolyte to be introduced into the main denuder viz. the first step denuder. t

The second step denuder may also comprise several compartments among whichthe amalgam not sufiiciently stripped of alkali metal in the first step denuder is distributed.

The accompanying drawings illustrate diagrammatically the principle of the invention. Figure 1 shows by Way of example a device comprising the compartments of a denuder as well as the flow ofthe electrolyte, of *rich amalgam, of partially decomposed amalgam, and of amalgam at the end of the decomposition. Figure 2 is similar to Figure 1 but completed by some modifications which ensure a greater flexibility of regulation in the working of the denuder." i a .As shown in the drawings the various compartments of the first step denuder 'l receive the dilute lye formed in the compartmentsi of the second step denuder; the source of rich amalgam is represented by 3, the partially decomposed amalgam is taken up by a pump 5, Whilst the mercury containing no amalgam is taken up by a pump 6'and led via 4 to the manufacture of alkali metal amalgam. The Water required for the manufacture of the lyes is taken from a tank 7 and the alkaline solution is concentrated as it passes from one compartment to the next one to the outlet 13.

According to Figure l, the rich amalgam withdrawn from 3 is distributed among the compartments 1 of the first step denuder by the pipes 8. There, it is rapidly decomposed in the presence of the relatively dilute lye coming from the second step denuder. After decomposition of the amalgam, the mercury flows through the pipes 9 and is takenfup by the pump 6 which delivers it to the apparatus producing rich amalgam 3-4. The amalgam penetrating the last compartments of the first step denuder meets there concentrated caustic lyes. When leaving these compartments it may not yet be sufliciently stripped of the alkali metal and in this case it flows through the pipes which deliver it to the pump 5. It is then returned to the second step denuder and distributed by the pipes 11 among the various compartments 2. The exhaustion in alkali metal is practically complete and the mercury leaving through the pipes 12 re- 3 joins the pump 6 which returns it, together with the mercury flowing through the pipes 9, to the device 34 producing amalgam.

Figure 2 shows a combination of the circulation circuits of electrolyte and amalgam; the references 1 to 13 designate the elements already mentioned in the description of Figure 1. According to the working conditions of the plants it may be useful to provide for the electrolyte of the second step denuder a recycling circuit. This would be useful, for example, when the amalgam delivered by the pump 5 is very poor in alkali metal, or when the lye produced in this denuder is still too dilute, or also in other cases. In anticipation of such a contingency, the lye leaving the said denuder may be taken up by the pump 14 and the flow of the recycled electrolyte is then regulated by the valve 15 whereas the total supply to be fed to the first step denuder is regulated by the valve 16. On the other hand, it'may be useful to dilute the amalgam delivered through the pipes 10. For this purpose there has been provided a valve 17 which establishes communication between the pipes 9 and it) and a valve 18 to establish communication between the pipes 9 and the pump 6. By operating these two valves the feeds of the pumps 5 and 6 are regulated.

The arrangements described are not limitative. It is possible to modify them without departing from the scope of the invention. Thus it is possible, for example, to use any type of second step denuder to complete the decomposition of the amalgam as long as the'caustic lye produced in this denuder forms the electrolyte entering the first step denuder. Under certain working conditions the amalgam may be completely decomposed in the compartments of the denuder containing the concentrated caustic lye; in this case the second step denuder may become superfluous. It is also possible to distribute the rich amalgam among the compartments 1 according to different deliveries from one compartment to the other, by arranging regulating valves in the pipes 8 and, if desired, 11.

Finally, it is possible to withdraw from the cycle the total or a portion of the dilute lye produced in the second step denuder.

We claim:

1. In a process for the manufacture of an alkil metal hydroxide solution of high alkali metal hydroxide concentration by decomposing the corresponding alkali metal amalgam in the presence of water, the steps which CO4 prise, establishing a plurality of confined decomposition zones including a first zone, a last zone, and a plurality of intermediate zones, said zones being inter-connected for continuous gravity flow of an aqueous medium from said first zone to said last zone by passing seriatim through all of said intermediate zones, continuously introducing water into the first zone and allowing said water to flow from said first zone through said intermediate zones Into said last zone, subdividing the amalgam to be decomposed into a plurality of independent streams corresponding in number to the number of said zones, simultaneously and continuously feeding each stream into only one of said zones, no two streams entering the same zone, and each amalgam stream flowing only through the zone into which it is introduced, wherein the amalgam in each stream is at least partially decomposed in each zone by contact with the aqueous medium therein, the concentration of alkali metal hydroxide in said aqueous medium thereby increasing in continuously passing successively from said first zone through said intermediate zones into said last zone, continuously removing the separate streams of the at least partially decomposed amalgam issuing from each of said zones, and removing alkali metal hydroxide solution from said last zone.

2. In a process for the manufacture of an alkali metal hydroxide solution of high alkali metal hydroxide concentration by decomposing the corresponding alkali metal amalgam in the presence of water, the steps which comprise, establishing a plurality of confined decomposition zones including a first zone, a last zone, a plurality of first intermediate zones, a plurality of second intermediate zones, and a plurality of third intermediate zones, said zones being inter-connected for continuous unidirectional flow of an aqueous medium from said first zone to said last zone by passing seriatim through said intermediate zones, continuously introducing water into said first zone and allowing said water to flow from said first zone through said intermediate zones into said last zone, subdividing fresh amalgam to be decomposed into a plurality of independent streams, corresponding in number to the sum of said second and third intermediate zones and said last zone, simultaneously and continuously feeding each stream of said fresh amalgam into only one of said second and third intermediate zones and said last zone, no two streams entering the same zone, and each amalgam stream flowing only through the zone into which it is introduced, wherein the amalgam in each stream is at least partially decomposed in each zone by contact with the aqueous medium therein, the concentration of alkali metal hydroxide in said aqueous medium thereby increasing in continuously passing successively from said intermediate zones into said last zone, continuously removing the separate streams of the at least partially decomposed amalgam issuing from each of said second and third intermediate zones and said last zone, subdividing .at least part of the amalgam issuing from said third intermediate zones and said last zone into a plurality of independent streams corresponding in number to the sum of said first zone and said first intermediate zones, simultaneously and continuously feeding each stream of said last-named amalgam into each of said first intermediate zones and said first zone, no two streams entering the same zone, and each amalgam stream flowing only through the zone into which it is introduced, continuously removing each stream of amalgam after it has passed through its respective zone, and removing alkali metal hydroxide solution from said last zone.

3. A process for the manufacture of an alkali metal hydroxide solution of high alkali metal hydroxide concentration as defined in claim 2, further comprising the step of recovering the aqueous medium issuing from the last of said first intermediate zones and recycling said medium to said first decomposition zone.

4. In a process for the manufacture of an alkali metal hydroxide solution of high alkali metal hydroxide concentration by decomposing, in the presence of water, the corresponding alkali metal amalgam issuing from an electrolysis zone wherein said amalgam is formed during the electrolytic decomposition of an alkali metal salt, the steps which comprise, establishing a plurality of confined decomposition zones including a first zone,'a last zone, and a plurality of intermediate zones, said zones being interconnected for continuous unidirectional flow of an aqueous medium from said first zone to said last zone by passing seriatim through all of said intermediate zones, continuously introducing the aqueous medium into the first zone and allowing said aqueous medium to flow from said first zone through said intermediate zones into said last zone, subdividing the amalgam to be decomposed into a plurality of independent streams corresponding in number to the number of said zones, simultaneously and continuously feeding each stream into only one of said 5 zones, no two streams entering the same zone, and each amalgam stream flowing only through the zone into which it is introduced, wherein the amalgam in each stream is at least partially decomposed in each zone by contact with the aqueous medium therein, the concentration of alkali metal hydroxide in said aqueous medium thereby increasing in continuously passing successively from said first zone through said intermediate zones into said last zone, removing the separate streams of the at least partially decomposed amalgam issuing from each of said zones, returning at least some of said last named amalgam to said electrolysis zone, and removing alkali metal hydroxide solution from said last zone.

5. In a process for the manufacture of an alkali metal hydroxide solution of high alkali metal hydroxide concentration by decomposing, in the presence of water, the corresponding alkali metal amalgam issuing from an electrolysis zone wherein said amalgam is formed during the electrolytic decomposition of an alkali metal salt, the steps which comprise, establishing a plurality of confined decomposition zones including a first zone, a last zone, a plurality of first intermediate zones, a plurality of second intermediate zones, and a plurality of third intermediate zones, said zones being inter-connected for continuous unidirectional flow of an aqueous medium from said first zone to said last zone by passing seriatim through said intermediate zones, continuously introducing water into said first zone and allowing said water to flow from said first zone through said intermediate zones into said last zone, subdividing said amalgam from said electrolysis zone into a plurality of independent streams, corresponding in number to the sum of said second and third intermediate zones and said last zone, simultaneously and continuously feeding each stream of said amalgam into only one of said second and third intermediate zones and said last zone, no two streams entering the same zone, and each amalgam stream flowing only through the zone into which it is introduced, wherein the amalgam in each stream is at least partially decomposed in each zone by contact with the aqueous medium therein, the

' concentration of alkali metal hydroxide in said aqueous medium thereby increasing in continuously passing successively from said intermediate zones into said last zone, continuously removing the separate streams of the at least partially decomposed amalgam issuing from each of said second and third intermediate zones and said last zone, subdividing at least part of the amalgam issuing from said third intermediate zones and said last zone into a plurality of independent streams corresponding in number to the sum of said first zone and said first intermediate zones, simultaneously and continuously feeding each stream of said last-named amalgam into each of said first intermediate zones and said first zone, no two streams entering the same zone, and each amalgam stream flowing only through the Zone into which it is introduced, removing each stream after it has passed through its respective zone, returning at least some of said last-named amalgam to said electrolysis zone, and removing alkali metal hydroxide solution from said last zone.

6. A process for the manufacture of an alkali metal hydroxide solution of high alkali metal hydroxide concentration as defined in claim 5, further comprising the step of recovering the aqueous medium issuing from the last of said first intermediate zones and recycling said medium to said first decomposition zone.

7. An apparatus for the manufacture of an alkali metal hydroxide solution of high alkali metal hydroxide concentration by decomposing a corresponding alkali metal amalgam in the presence of water, comprising means defining a plurality of confined vertically-spaced-apart decomposition compartments including a first compartment, a last compartment, and a plurality of intermediate comparments, said compartments being inter-connected for continuous unidirectional flow of an aqueous medium from said first compartment to said last compartment by passing seriatim through all of said intermediate compartments, means for continuously introducing Water into said first compartment and causing said water to flow from said first compartment through said intermediate compartments into said last compartment, conduit means for simultaneously supplying the amalgam to be decomposed in a plurality of independent streams corresponding in number to the number of said compartments into each of said compartments, with no two streams of amalgam entering the same compartment, whereby the path of the amalgam through the apparatus is subdivided, means for withdrawing each stream of amalgam by isolating it from the water after it has passed through its respective compartment, means for removing at least partially decomposed amalgam from each of said compart ments and passing it into the next succeeding compartment, and means for removing alkali metal hydroxide solution from said last compartment.

8. An apparatus for the manufacture of an alkali metal hydroxide solution of high alkali metal hydroxide concentration as defined in claim 7, wherein said intermediate compartments include a plurality of first intermediate compartments, a plurality of second intermediate compartments, and a plurality of third intermediate compartments, said apparatus further comprising means for transferring the at least partially decomposed amalgam from said third intermediate compartments to said first compartment and said first intermediate compartments, and means for withdrawing some of the aqueous medium issuing from the last of said first intermediate compartments and returning said medium to said first compartment.

References Cited in the file of this patent UNITED STATES PATENTS 586,635 Stormer July 20, 1897 2,083,648 Gorke June 15, 1937 2,588,469 Basilewsky Mar. 11, 1952 2,610,908 De Prez et al Sept. 16, 1952 2,732,284 Sakowski Jan. 24, 1956 FOREIGN PATENTS 115,776 Australia Aug. 25, 1942 

1. IN A PROCESS FOR THE MANUFACTURE OF AN ALKIL METAL HYDROXIDE SOLUTION OF HIGH ALKALI METAL HYDROXIDE CONCENTRATION BY DECOMPOSING THE CORRESPONDING ALKALI METAL AMALGAM IN THE PRESENCE OF WATER, THE STEPS WHICH COMPRISE, ESTABLISHING A PLURALITY OF COMFINE DECOMPOSITION ZONES INCLUDING A FIRST ZONE, AT LAST ZONE, AND A PLURALITY OF INTERMEDIATE ZONES, SAID ZONES BEING INTER-CONNECTED FOR CONTINUOUS GRAVITY FLOW OF AN AQUEOUS MEDIUM FROM SAID FIRST ZONE TO SAID LAST ZONE BY PASSING SERIATIM THROUGH ALL OF SAID INTERMEDIATE ZONES, CONTINUOUSLY INTRODUCING WATER INTO THE FIRST ZONE AND ALLOWING SAID WATER TO FLOW FROM SAID FIRST ZONE THROUGH SAID INTERMEDIATE ZONES INTO SAID LAST ZONE, SUBDIVIDING THE AMALGAM TO BE DECOMPOSED INTO A PLURALITY OF INDEPENDENT STREAMS CORRESPONDING IN NUMBER TO THE NUMBER OF SAID ZONES, SIMULTANEOUSLY AND CONTINUOUSLY FEEDING EACH STREAM INTO ONLY ONE OF SAID ZONES, NO TWO STREAMS ENTERING THE SAME ZONE, AND EACH AMALGAM STREAM FLOWING ONLY THROUGH THE ZONE INTO WHICH IT IS INTRODUCED; WHEREIN THE AMALGAM IN EACH STREAM IS AT LEAST PARTIALLY DECOMPOSED IN EACH ZONE BY CONTACT WITH THE AQUEOUS MEDIUM THEREIN, THE CONCENTRATION OF ALKALI METAL HYDROXIDE IN SAID AQUEOUS MEDIUM THEREBY INCREASING IN CONTINUOUSLY 